I need to isolate a room to play and avoid disturbing neighbours. Since other musicians may have the same problem and experience, would asking it here be on topic?
I think it would be on topic here, because it falls under the 'practice' aspect of the site in my opinion. How to practice without annoying your neighbors is something many musicians have to think about and they have to find solutions. Similarly, under the same category falls questions like 'I need to record at home and I need to know how to isolate the room etc' (not so broad though).
So, I would say that it is on topic, as long as it is not broad, like Dave said.
"How do I acoustically isolate my room?" seems too broad and underspecified to me.
There have been well-received questions that only go so far as to specify the type of instrument involved, e.g. this one about electric guitar (and probably some about isolating drums that I haven't found), so at least historically, with this level of focus it would be on topic. Personally, I'd like to see even more specification of the detailed goals and constraints than the linked question.
I can envision that there will be a bit of a fuzzy boundary between where the musical-acoustics aspect of the question is weak enough that the question is more of a mechanical/construction question rather than one related to musical performance.